One of the most misunderstood aspects of US tariffs is stacking — the way tariffs from different legal authorities add up on the same import. After the SCOTUS ruling struck down IEEPA tariffs and replaced them with 10% Section 122, and with steel/aluminum doubled to 50%, the stacking math has changed significantly. Understanding tariff stacking is essential for accurate cost calculations.
What Is Tariff Stacking?
Tariff stacking occurs when multiple tariff programs apply simultaneously to the same import. Each tariff comes from a different legal authority and is assessed independently. The US currently has three main tariff regimes that can stack: Section 122 tariffs (10% flat for all countries, effective Feb 24, 2026), Section 232 tariffs (50% steel/aluminum, 50% copper, 25% autos, 25% semiconductors), and Section 301 tariffs (7.5-100% on Chinese goods). When a product falls under multiple programs, you pay all of them — they don't replace each other, they add up.
Real-World Stacking Examples (Post-SCOTUS)
Chinese steel imports: 10% Section 122 + 50% Section 232 + 25% Section 301 = 85% total duty. Chinese electric vehicles: 10% Section 122 + 100% Section 301 = 110%. Chinese solar panels: 10% Section 122 + 50% Section 301 = 60%. Vietnamese steel: 10% Section 122 + 50% Section 232 = 60%. German automobiles: 10% Section 122 + 25% Section 232 auto = 35%. Japanese steel: 10% Section 122 + 50% Section 232 = 60%. UK steel: 10% Section 122 + 25% Section 232 (EPD rate) = 35%. These examples show why the 'announced rate' rarely tells the full story.
Anti-Stacking Exception: Canada/Mexico Autos
There is an important anti-stacking rule for automobiles from Canada and Mexico: goods subject to the Section 232 automobile tariff (25%) are NOT also subject to the Section 122 tariff. This means a Canadian-made car pays 25% (Section 232 auto) rather than 25% + 10% (Section 232 + Section 122). This exception exists because the automobile industry is deeply integrated across North America, and double-taxation would be prohibitive. USMCA-qualifying vehicles can potentially avoid the Section 232 tariff entirely.
Anti-Stacking Exception: Canada/Mexico Steel/Aluminum
A second anti-stacking rule applies to steel and aluminum from Canada and Mexico: USMCA-qualifying goods are exempt from Section 232 steel/aluminum tariffs (50%). Non-USMCA Canadian steel pays both 10% Section 122 + 50% Section 232 = 60%. Note: this stacking applies to most countries. The UK is the only major exception with a reduced 25% Section 232 rate under the Economic Prosperity Deal.
Why Effective Rates Differ from Announced Rates
When tariff rates are discussed in the media or in trade negotiations, the number cited is usually just one component. 'A 10% tariff under Section 122' doesn't include Section 232 (50% on steel) or Section 301 (25-100% on Chinese goods). Always calculate the full stacked rate for your specific product and country combination. The difference between the headline rate and the effective rate can be 25-75 percentage points or more.
Key Takeaway
Tariff stacking means your actual duty rate is often 2-8x the headline number. China steel at 85%, Chinese EVs at 110%, and Vietnamese steel at 60% are common examples. Always check all three tariff programs (Section 122, Section 232, Section 301) for your product. USMCA and UK EPD offer limited relief on certain products.
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